The present invention relates to four channel stereophonic sound equipment and more especially to such four channel equipment of the "matrix" type.
In a concert hall, a listener receives sounds from, for example, an orchestra directly and additionally receives sounds reflected from the walls, ceiling and other structures within the concert hall. The first step in attempting to reproduce the concert hall environment in a realistic manner was to record or transmit two separate sound channels representing respectively the sounds coming from the left and right sides of the concert hall stage. This system is fairly realistic in its sound reproduction, however, there have been recent attempts to provide audio systems having more than two channels so as to provide a listener with an even more realistic sound reproduction. These recent attempts have typically been four channel systems using one of two very different approaches.
One approach to providing a four channel stereophonic sound system is to simply add two additional channels to existing two channel stereophonic schemes. In the well known two channel stereophonic systems, the outputs of two physically separated microphones are recorded on two independent tracks in a tape recorder or analogously transmitted or recorded on a long playing record, and subsequently each independent track or channel is amplified and the signal to two physically separated speakers to recreate the original sound environment. It is clearly possible to provide for example, a four channel tape recorder and using four physically separated microphones to record their individual responses and subsequently using four separate amplifiers and speaker systems to reproduce in a four channel manner, the original sound environment. This approach is called "discrete" four channel stereo and while it gives extremely realistic sound reproduction, it is quite costly and plagued with numerous technical problems. For example, it is well established procedure to record two distinct channels on a phonograph record, however, no satisfactory system for recording four distinct channels on such records has yet been established. A discrete four channel stereo system requires four separate audio amplifiers and of course the existing allocation of radio frequencies renders four channel radio transmission difficult and costly.
Another approach to providing four channel stereophonic sound, known as the matrix approach, begins with the four different electronic signals from four separated microphones but mixes these four signals so as to provide just two channels of information for recording or transmission. The two thus mixed channels are of course quite compatible with existing equipment and readily recorded or transmitted. These two mixed channels are subsequently decoded in some manner to approximately reproduce the original four channels of information. Known accurate decoding techniques require four audio amplifiers in the reproduction equipment since the decoding is performed prior to power amplification.
In contradistinction, the present invention provides a matrix type four channel stereophonic sound system requiring only two audio amplifiers and effects the decoding subsequent to amplification in a passive network at the terminals of the four speakers.